Valve gap: what happens to it after engine warms up?
 

I'm adjusting my valves as per the backside method. I'm using the 0.0025 as the go and the 0.0030 as the no go. As I adjust, I try to make the adjustment as tight to the 0.0025 as possible. Once it's all done and I get the engine running, what happens to that gap? Metals expand as they heat up, but different metals expand at different rates. I don't know what metals are used in the adjustment screw and other components, nor do I know anything about expansion rates of metals. So does the gap tighten some more or loosen? Should I adjust closer to 0.0030 or closer to 0.0025?

That is interesting, Vets?

The gap tightens. (edit: Sorry, This is wrong, as below others have reported as measuring a wider gap on warm engines, read this whole thread for more information.) Henri, as long as you can slide the 0.0025 in and not the 0.003 you are as close as you ever need to be. It has been my experience that over time the gap will tighten rather than loosen.

Many people have success with different methods, I like to adjust using the normal method, and then confirm using the backside method. When I do this I often find that I can still get the 0.0030 to go in. When I adjust a bit tighter I can no longer get the 0.004 in between the lobe and the foot.

I would recommend that you figure out how to do the normal method measuring between the lobe and the foot with the 0.004 first and once you figure that out then go on to the backside method if it is simpler for you.

You will see that after a few times of doing this you will be able to tug on the them to feel the gap. Then you can tell the difference between loose and tight.

Here is one of the best threads for the backside method: http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/390457-backside-valve-adjustment-my-thoughts.html

The valve stem gets longer which is why most engines have a bigger clearance on exhaust valves.

Really?

As we all know the valves and rockers sit in an alloy carrier.
Thinking about it... As the alloy expands the gaps would open up not tighten although I've never measured a hot engine. (alloy expands more than steel)

It would be good to get an accurate measurement of how much the gaps change.

http://forums.pelicanparts.com/uploa...1302158091.gif

That's what I have always read, learned, known.

I think in porsche engines it increases because when the engine is hot usually you can hear more tapping noise. However, that is not a general rule. In fact depending on the head configurations in some engines the play increases and in others it get tighter.
luca

but have you measure it?

When warm? No, I haven't. (or I would have mentioned that for sure :))

In one car I had I'm sure the gap would get wider when hot. The car would start with 2-3 cylenders (out of 4 :) ) when cold and then run normally after a couple of minutes, but will start perfectly when hot. Resetting the gap at cold solved the issue completely. Same happens for example with modern suzuki GSXR engines. The gap does increase when hot.
On other engines it is rather common instead to have troubles starting the engine when it's hot because the valve has no more play and remains open. Ask any motocross rider (especially riding a honda CFR) what a big difference makes to start the engine hot if the valves are not properly gapped.
luca

We have discussed this several times, here is a good thread:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/519852-temperature-effect-valve-adjustments.html

All of us who have actually measured the hot clearance on a 911 engine agree that the gap widens when hot. This is consistent with my experience with many other OHC and DOHC aluminum engines. The valve gap is not about thermal expansion, it is about giving the valve enough seat times to give off heat.

Hi Paul,

I remember that thread it made my head hurt. Can you summarize your theory here? Was it that because the cam carriers expand at a higher rate that this moves the rockers and cam farther away, which increases the gap?

Shane

Shane,

No theory involved, just get the engine up to temperature and take a few measurements. All the aluminum head engines I have measured hot have wider clearances. The cause ? My guess would be that the surrounding aluminum expands at a much higher rate than the ferrous material. I put this in the "old wives tale" box, along with the idea that noisy valves just need adjustment. My experience is, that valve clearances close down with wear, adjusting them makes them "more noisy" , and the noise usually comes from wear in the guides, rockers, tappets or tappet guides (depending on the engine design) , not variations in the running clearances.

This makes sense to me, from experience, and I've talked to a couple local guys who do a lot of work like this. One of those men (when I asked him this very question) said, "Not necessarily...", to the question of things tightening up when hot.
I had a "ticker", he fixed it. Previous to his work, it was fine when "cold" but ticking when warm. Now it is quiet at all temps (normal racket back there is all), and has been for thousands of miles.....odd, perhaps, but true....I think....:)

It gets wider. I've measured it using the "standard" method using an .004" feeler.

Try it sometime.

Are you saying that he decreased the cold gap?

When the engine heats to operating temperature, the heads, cam carriers, valve, cam lobes, rockers all expand, but the gap between the valve stem and the adjustment screw base increases?

Quote: "Are you saying that he decreased the cold gap?"
Yep, but only within reason. "Porsche 911" he said. It was just a bit loose; anything over .004, or even a little "loose"/very little drag at .004 is a big no-no in my mind...I've read before of guys who go tighter than .004 (but very close) cold with no problems, after years of hard use....but hard to say what the truth is....controversy exists it seems?

This site seems to explain it better: (read the last paragraph about the differences between push-rod and OHC)

Valve Clearance

I think I am starting to understand.

Great! I hope to also, lol. I'll read that stuff....

maby true but that defeats the purpose of the adjustment. Unless your just going for noise reduction.

I have measured the gap cold at .004 and hot at .007 many years ago


regards

Two comments....

1.) It amazes me that someone posts "categorically" that it goes one way.....when in fact it goes the other way. Leads to dangerous lasting posts for the archives for others to depend on....glad to see others responding who have actual experience stating which way it truly goes ( opposite).

2.) Spec has always been 0.004" ( actually 0.1 mm if you give more credence to the metric homeland measurement, or 0.0039"). Look under many 911 engine lids on the emissions sticker.....there is a lot more acceptable "give" than we think. The spec stated there is 0.004", (+/-) 0.002". So it seems the acceptable range is 0.002" to 0.006"

Lastly, my feeling is this. Designers are often forced into a situation where a numerical measurement has to be given for spec purposes, but that it may not be "critically" important. This may be one of these cases. The core concept is that there should be a "scooch" of air-gap clearance measured cold...and that it cannot be "closed". Then the fun begins....."what number should we show as "spec" ... ???

Yeah, yeah.... I know, tolerancing closer to open vs cold affects true valve timing overlap/lift/duration...and that it directly affects that last 1 hp you may be chasing....but I don't think we're talking this point here.

And then someone invented hydraulic lifter that run at zero clearance.

the gap will widen a small bit after it heats up nice and toasty,..that's been the info I've rx'd from the P-wrenches I know.

Best,

Doyle

I know that some 24hr race engines widen the valve lash on the exhaust valves. I think this question could have different answer for exhaust vs intake valves. In a race engine running under very hi-loads the exhaust valve surely heats up. I suspect that a super hot valve will measure longer than a cool one. But that's just a guess.

Porsche usually mandates lash settings of .006 inlet and .008 exhaust Cold settings

Say what?

Sorry Bourbon fingers forgot to say on racing engines from 906 to current Cup .006 on intake and .008 exhaust. cam ramp+ expansion


regards

Yeah, and the "hot check" info is really great. ...but what if the hot measurement doesn't tell the whole story?

We are talking engine shut down measuring of valve lash, of a heat-soaked engine, right? That, however, is different than measuring the relative thermal expansions (the gap) when that exhaust valve is punching thru exhaust gasses at full load and 6000rpm.

All I'm saying is there are transient temperature gradients which you won't see with a stopped engine and check of the valve gap. ...the thermal mass of the valve is small (compared to the cylinder head) :)

I should add... That minimum .002" would not be specified if valve lash "always increased with temperature." Because IF "valve lash always (and only) increased with temperature" then you could simply adjust them to net zero (cold) and not immediately burn valves on first run. --I expect that valves would burn pretty quickly with zero gap. ..don't try this at home. ;)